And other fun facts about the Big Apple’s sort of randomly lettered subway lines.

New York City does not have the busiest subway system (that would be Tokyo), or necessarily the prettiest (Moscow’s are like museums), but in my totally biased opinion, it does have one of the most navigable systems. In Manhattan, the trains follow the grid system, and even when they don’t in the outer boroughs, stations are named after the streets that they’re on.

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There is, however, one peculiarity: while the subway trains are named after letters of the alphabet, several letters are missing. There’s no H, I, K, O, P, T, U, V, W, or Y train. So why not?

Mental Floss has some answers. For some, it simply has to do with infrastructure changes that occur as the Metropolitan Transit Authority discontinue trains and then add new ones. The T train, for instance, was absorbed by construction changes into the B train, and then the D train. Because the MTA recycles old letters, there will be a newly minted T train in 2016, when construction on the Second Avenue Subway finishes.

For the others, however, the reasoning has to do with city signage and service design. Because vowels often sound too similar to commonly used words, it’s thought that they would have caused confusion for passengers, according to Hannah Keyser over at Mental Floss:

The letters I and O were never used for trains because of their visual similarities to the numbers 0 and 1 and the use of both alphabetical and numerical designations in the New York Subway system.

U and Y were eliminated from consideration because they are homonyms with actual words–namely, “you” and “why.” Apparently, the founders of what is now the MTA did not find the potential for “Who’s on First”-style confusion to be nearly as humorous as Abbott and Costello did.

The lonely outlier is the letter P. There’s little concrete evidence to explain why it’s been neglected all these years. One theory? The MTA isn’t tickled by bathroom humor:

There are several stories of almost-P trains…it is fun, if not entirely likely, to consider the theory that the P homonym was simply too immature for polite commuter conversation.